Advanced filtration membranes using chitosan and graphene oxide

ABSTRACT

A composition of five parts by mass of chitosan and one part graphene oxide is suspended in water. The composition may be used to form filtration layers of any size or shape and may be reinforced by additional layers. The composition may be used to construct a large filtration apparatus of any size or shape and may be used to form highly resilient, antimicrobial structures and surfaces for a variety of applications.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by the Government ofthe United States of America for governmental purposes without paymentof royalties.

FIELD OF INVENTION

This invention relates to the field of water filtration devices, andmore specifically to a highly resilient graphene filter.

BACKGROUND OF THE INVENTION

The U.S. Army Corps of Engineers Research and Development Center (ERDAC)has a mission to identify technologies to make clean water available andaddress global water shortages.

In furtherance of this mission, researchers at ERDAC's EnvironmentalLabs have sought to identify superior materials from which more durablefiltration devices can be constructed. It is a problem known in the artthat current water filtration devices experience rapid deterioration andquickly wear out in harsh, corrosive chemical environments and duringthe process of desalination.

The cost of frequently replacing filtration devices is a significanteconomic barrier to bringing clean water to an estimated 63 millionpeople that currently do not have access to it.

Various attempts have been made in the art to produce more durable,longer-lasting water filtration devices. Durable metals, are notsuitable for constructing thin filter membranes that do not disruptwater flow.

In contrast, graphene is a material that is thought to be the strongest,thinnest and lightest material available and which can be used to formmembranes.

However, it is a problem known in the art that graphene oxide filters,although highly durable, become deformable and unstable when immersed inwater while used for the production of large filtration devices. Thisdeformation allows smaller contaminate molecules and salts to flowthrough the pores of the filters, rendering them ineffective.

There is an unmet need for graphene oxide filters that exhibit thestrength of life cycle of graphene, but which are not vulnerable todeformation and swelling as the size of the filtration membrane isincreased.

BRIEF SUMMARY OF THE INVENTION

The invention is a chitosan graphene oxide (CSGO) filtration membranecomprised of five parts by mass of chitosan and one part graphene oxidesuspended in water. The composition may be of any size or configuration.It is incubated under a slight vacuum. As the water evaporates, amembrane is cast into the mold.

The method for making the CGSO filter includes protonating a quantitychitosan to create protonated chitosan; evaporating a solution on asurface to create a membrane having a target thickness; and placing saidmembrane between two layers. In one embodiment, said quantity ofchitosan and said quantity of graphene have a 5:1 ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an exemplary method for producing a highlyresilient chitosan graphene oxide (CSGO) filter.

FIG. 2 (Prior Art) illustrates the structural makeup of graphene oxideand an exemplary oxide group.

FIG. 3 illustrates an exemplary chitosan reaction to produce protonatedchitosan.

FIG. 4 illustrates the cross-linking of graphene oxide and chitosan.

FIG. 5 illustrates an exemplary layered structure of a CGSO filter.

FIG. 6 illustrates a CGSO filter which is spiral shaped.

FIG. 7 (Prior Art) illustrates a channel structure formed in a CSGOmembrane during the evaporation process.

FIG. 8 is a graph illustrating the improved flow rate of a GSGO filterover a graphene oxide filter.

FIG. 9 is a chart illustrating the the flux rate of a CSGO membrane ascompared to other membrane compositions.

FIG. 10 is is a chart illustrating the comparative performance of a CSGOfilter to a graphene oxide filter and an nano-filtration member.

TERMS OF ART

As used herein, the term “cross-linking” means binding two materials atthe molecular level.

As used herein, the term “CSGO” means chitosan graphene oxide.

As used herein, the term “disk-shaped” means a disk-like structure of afiltration membrane.

As used herein, the term “filtration membrane” means a thin layer ofsemi-permeable material that separates molecules, particles orsubstances that pass through it.

As used herein, the term “highly-resilient” means durable and able to beused for various applications.

As used herein, the term “membrane support layer” means a structure forsupporting a CSGO filter.

As used herein, the term “spiral wound” is a spiral-like structure ofthe filtration membrane.

As used herein, the term “target thickness” means a desired thickness tobe achieved for creation of a filtration membrane relative to itsintended use.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an exemplary method for producing a highlyresilient chitosan graphene oxide (CSGO) filter.

Step 1 is the step of protonating a quantity of chitosan to createprotonated chitosan. In this step, a quantity of chitosan is mixed withan acid to create positively charged (protonated) chitosan. The acid isan organic acid.

In one exemplary embodiment, acetic acid is used to protonate thecompound. In various embodiments, other acids may be used, including butnot limited to Acetic Acid, Malic, Succinic Acid, Glycolic Acid, OxalicAcid, Adipic Acid, Citric Acid, Formic Acid, Carboxylic Acid, SulfonicAcid, Muriatic Acid and Tannic Acid.

Step 2 is the step of mixing a quantity of graphene oxide with saidprotonated chitosan to create a solution. It is a problem known in theart that Chitosan is not water soluble. This step produces a homogeneousmixture by rendering the chitosan water soluble, which may then be mixedwith graphene oxide. In the exemplary embodiment shown chitosan andgraphene oxide must be combined in a 5:1 ratio.

Step 3 is the step filling a molding structure with a quantity of saidsolution. It is necessary to make sure the mixture is well stilled tocreate a homogeneous composition.

Step 4 is the step of evaporating the solution on a surface to create amembrane having a target thickness. Evaporation of the graphene oxideand chitosan composition may be accomplished at room temperature. Duringthe evaporation process, cross-linking of the graphene oxide andchitosan occurs.

In various embodiments, convective air flow may be employed toaccelerate the evaporation process. In other embodiments, heat may beused to expedite the evaporation process. Reducing air pressure willalso expedite the . evaporation process.

The target thickness is a function of a molecular size of thecontaminant versus that of the filtration target along with the criteriafor filtering contaminates selectively. In certain embodiments, this isgenerally within a range of 5 to 200 microns. Thinner membranes allowfaster flux and flow but are less resilient than thicker membranes.

In the exemplary embodiment shown, a mold is used to form the filtrationmembrane. In one exemplary embodiment, the mold is a smooth,non-absorbent sheet with edges to retain the composition. Thecomposition is poured into the mold and allowed to evaporate.

Step 5 is the step of placing said membrane between two layers. Themembrane in the previous step may be sufficient for one-directionalfluid flow. For example, treatment of the filters by strong base (pH>10)or thermally may result in stable performance without the use ofadditional layers or filters. However, it may be desirable to provideadditional support for the filtration membrane for bi-directional waterflow (cross-flow) by affixing layers to the filtration membrane.

The filtration membrane may be placed between or in contact with one ormore membrane support layers of different material. The support layerreinforces the filtration membrane.

The CSGO filter resulting from the above described method may be scaledfor any application. It may be used in situ, adapted for equipment orplaced within various pipes, valves, tanks, receptacles and other waterand gas barriers and structures.

In various embodiments, the CSGO filter may be die cut, punched orextruded. The die cut filter may be used for gas filtering applicationsas well as for water filtering. Exemplary embodiments include but arenot limited to gas separations, CO2 capture, visible lightphotocatalysis, etc. The die cut filter may further be used for point ofuse water purification, antimicrobial surfaces and coatings, etc.

The filter may also be used for antimicrobial purposes and is lesssusceptible to fouling. Anti-microbial activity was tested bacterialgrowth in solution and on the membrane surface, respectively. The CSGOmaterial inhibited bacterial growth in solution relative to cellulosefiltration membranes and commercial reverse osmosis membranes asmeasured using optical density (OD 600 nm). In addition, the CSGO filtermay inhibit biofilm formation on the membrane surface as determined bygrowth on agar plates.

Other uses for large scale graphene oxide filters resulting from theforegoing may include drinking water treatment in large water treatmentplants, industrial wastewater treatment, water reuse applications,deployable treatment systems, removal of salts in individual groundwatertreatment systems, treatment of groundwater using pump and treatsystems, supplementary home water treatment for the removal ofcontaminants (e.g., lead), etc.

Still yet, in other embodiments the graphene oxide filters can beconfigured for specialized use within equipment, such as for arefrigerator filter or as a filter to remove sediment, microorganisms,impurities and the like within a laboratory device. Moreover, thegraphene oxide filters can be used for Water softening and for mobilepurification for individual water, such as camping, deployed militaryforces, disaster response, etc.

FIG. 2 (Prior Art) illustrates the structural makeup of graphene oxideand an exemplary oxide group. Graphene is impermeable and membranesconstructed from it are not known to be used commercially to producefiltration membranes The introduction of oxides results in a permeable,cross-linked compound.

FIG. 3 illustrates an exemplary chitosan reaction to produce protonatedchitosan. Oxide groups are negatively charged. Protonating the chitosanresults in a positively charged bio-polymer that produces electrostaticbinding between the oxides and chitosan.

FIG. 4 illustrates the cross-linking of graphene oxide and chitosan. Thechitosan and graphene oxide cross-link by electrostatic bonding orcovalent reaction, depending on the oxide group selected.

FIG. 5 illustrates an exemplary layered structure of a CGSO filter. Inthe exemplary embodiment, the CSGO filter 100 includes a CSGO filtrationmembrane 10 that is placed between support layers 11 a and 11 b. Layers11 a and 11 b support the filtration membrane 10 to prevent swelling anddeformation.

In the exemplary embodiment shown, the CSGO filtration membrane 10 isplaced between two nitrocellulose filters, which provide stability in across flow system. Nitrocellulose is selected because it non-reactivewith contaminants passing through and is scalable. Other materials withsimilar characteristics may be substituted.

In other embodiments, the support layers 11 a and 11 b may be comprisedof different material such as paper, glass wool and permeable plastic.

FIG. 6 illustrates a CGSO filter which is spiral shaped. Visibleelements of the spiral shaped CSGO filter 102 includes the CGSOfiltration membrane 10 and support layers 11 a and 11 b. As noted,layers 11 a and 11 b are constructed of a permeable material whichsupports cross-flow motion of passing fluids, gases, etc.

The spiral shaped CSGO filter 102 may also include a spacing layer 12for. The spacing layer 12 is an optional layer used in the spiralembodiment to create additional spacing for water flow betweenrespective support layers 11 a and 11 b.

In the exemplary embodiment, the spiral shaped CSGO filter 102 furtherincludes a clean water collection pipe 16. The clean water collectionpipe is a structure that allows clean, treated water to be collected anddispersed away from the filter.

FIG. 7 (Prior Art) illustrates a channel structure formed in a CSGOmembrane during the evaporation process. Graphene oxide has a flakeystructure which must be cross-linked to form a competent membrane. Theflakes form irregular channels 80 a and 80 b through which watermolecules 82 a, 82 b and 82 c can pass. Contaminant molecules 84 a and84 b cannot pass though the channels 80 a and 80 b.

FIG. 8 is a graph illustrating the improved flow rate of a GSGO filterover a graphene oxide filter. Improved flow is to attributable toimproved channel structure, where the channel structure is slightlywidened because the chitosan increases the space between the chitosanflakes. The chitosan improves the structure of the channels formed inthe membranes during the evaporation process. Resultantly, these factorsincrease the rate of water flow without compromising contaminantrejection.

FIG. 9 is a chart illustrating the the flux rate of a CSGO membrane ascompared to other membrane compositions. The water flows more freelythrough the membranes because of the addition of the chitosan.

FIG. 10 is a chart illustrating the comparative performance of a CSGOfilter to a graphene oxide filter and an nano-filtration member. TheCSGO filter exhibited a higher flux rate as compared to a graphene oxidemembrane and reverse osmosis membranes.

The exemplary chart reflects test results for methelyne blue. Themembrane was tested, for example, on treatment of cesium and ranged from25 to 100%. Malathion removal ranged from 10 to 51%.

In this exemplary embodiment, the CSGO membrane was further testedagainst the following compounds in addition to methylene blue: cesiumchloride, methyl orange, malathion and insensitive munitions (IM)formulation IMX-101. For each compound, removal rates are shown todegrade over the duration of the experiment. However, removal neverceased.

When using CSGO membranes prepared with granular graphene oxide (GO),methyl orange was initially removed entirely when fed at a concentrationof 10 mg/L. The mechanism for removal appeared to be both adsorption andrejection for these membranes. For CSGO membranes prepared withnanofiltration GO, the initial removal was approximately 85% when fed atconcentration of 10 mg/L. For these membranes, the dominant removalmechanism appeared to be adsorption.

For both membranes, the removal rate decreased over time. The fluxrelated to the granular CSGO membrane varied between 1.5 LMH and 2.1LMH; the flux for the nanofiltration CSGO membranes varied mostly withina range of 0.5 LMH and 0.6 LMH.

Malathion was initially removed at a rate of 80% for an inletconcentration of 100 mg/L and 75% for an influent concentration of 1000mg/L. These removal results were identical to those measured fornanofiltration (NF) membranes. For both inlet concentrations, the fluxfor the CSGO membranes was 2 L/m²-hr (LMH). CSGO membranes preparedusing granular graphene oxide (GO) were used for these experiments.

IMX-101 contains three major compounds, including nitrotriazolone (NTO),2,4-dinitroanisole (DNAN), and nitroguanidine (NQ). A solutioncontaining 600 mg/L of NTO, 150 mg/L of NQ, 9 mg/L of DNAN, and 7 mg/Lof 2,4,6-trinitrotoluene (TNT) was fed to CSGO membranes prepared withgranular GO; TNT was included as a traditional munitions compound forcomparison. These membranes initially removed 100% of the TNT, 100% ofthe DNAN, 65% of the NTO, and 5% of the NQ.

The removal rates of TNT and DNAN exceeded those of reverse osmosis (RO)membranes used for comparison. The removal rates of NTO and NQ were at adeficit compared to RO membranes, however. The removal rates of NTO,TNT, and DNAN exceeded those of NF; the removal rate of NQ wasessentially identical between CSGO and NF membranes. The flux of thissolution related to CSGO membranes was 1.5 LMH.

What is claimed is:
 1. A method of making a highly-resilient fluid andgas apparatus comprised of the steps of: protonating a quantity chitosanto create protonated chitosan; mixing a quantity of graphene oxide withsaid protonated chitosan to create a solution; filling a moldingstructure with a quantity of said solution; evaporating a solution on asurface to create a membrane having a target thickness; and placing saidmembrane between two layers.
 2. The method of claim 1 wherein saidquantity of chitosan and said quantity of graphene have a 5:1 ratio. 3.The method of claim 1 wherein said quantity of chitosan and saidquantity .of graphene have a ratio between 4:1 and 6:1.
 4. The method ofclaim 1 in which the step of protonating includes the step of creating ahomogeneous mixture by rendering the chitosan water soluble.
 5. Themethod of claim 1 wherein said layers are cellulose layers.
 6. Themethod of claim 1 wherein said layers are comprised a material selectedfrom a group consisting of paper, glass wool and permeable plastic. 7.The method, of claim 1 wherein the step of protonating further includescombining said quantity of chitosan with an acid.
 8. The method of claim7 wherein said acid is selected from a group consisting of Acetic Acid,Malic, Succinic Acid, Glycolic Acid, Oxalic Acid, Adipic Acid, CitricAcid, Formic Acid, Carboxylic Acid, Sulfonic Acid, Muriatic Acid andTannic Acid.
 9. The method of claim 7 wherein said acid is organic. 10.The method of claim 1 which further includes selecting a targetthickness which is a function of a target rate of contaminant rejectionand target flow rate.
 11. The method of claim 1 which further includesplacing two or more of said membranes in serial configuration toaccomplish a multi-staged filtration process.
 12. A filtration apparatuscomprised of: a membrane formed from an evaporated chitosan and graphenesolution, wherein said membrane has a thickness between 10 microns and200 microns.
 13. The filtration apparatus of claim 12 wherein saidmembrane has a thickness of approximately 6 microns and a malathionfilter has a thickness of approximately 40 microns.
 14. The filtrationapparatus of claim 12 wherein said apparatus has a spiral woundstructure.
 15. The filtration apparatus of claim 12 wherein saidapparatus has a disk shape spiral wound structure
 16. The filtrationapparatus of claim 12 which has a rectangular shaped structure.
 17. Thefiltration apparatus of claim 12 which has a shape which corresponds tothe inner surface of a structure.
 18. The filtration apparatus of clam12 wherein said filtration apparatus forms a housing.
 19. The filtrationapparatus of claim 12 wherein said apparatus is a gas and waterfiltration apparatus having a spiral wound shape.
 20. The apparatus ofclaim 12 wherein said apparatus includes a malathion filter having athickness of approximately 40 microns.